In general, a construction machine such as an excavator may perform work for flattening the ground while moving a bucket forward and rearward. In order to perform the work for flattening the ground, a worker needs to precisely control motion of the boom and the bucket so as to constantly maintain a load applied to the ground surface by the bucket.
Therefore, a degree of fatigue felt by the worker is inevitably high in order to perform the work for flattening the ground.
In addition, in a case in which the boom is not precisely controlled during the work for flattening the ground, force applied to the ground surface by the bucket is very large, such that the bucket may be buried too much into the ground surface. On the contrary, in a case in which force applied to the ground surface by the bucket is very small, the work for flattening the ground cannot be properly performed.
Meanwhile, the construction machine also uses an optional device such as a breaker by substituting the bucket. The breaker is an optional device for breaking rocks, a paved road, and the like, and needs to apply a predetermined force to an object to be broken.
However, when the breaker performs work, a reaction in which the boom bounds upward at the moment when the breaker breaks the object to be broken occurs. Therefore, the worker needs to more precisely control the boom and the breaker.
Recently, researches on a configuration that allows the bucket to apply a predetermined force to an object such as a ground surface or a rock using a weight of the boom are being conducted in order to resolve the aforementioned inconvenience. Particularly, because the boom bounds upward when the breaker performs work, work characteristics need to be considered even though the weight of the boom is used.
There is Patent Literature 1 that is previously filed by the applicant of the present disclosure, and laid open.
However, a boom cylinder control circuit disclosed in Patent Literature 1 has the following problem.
A large amount of working fluid is discharged from a boom cylinder, and a small amount of working fluid is discharged from a spool of a boom control unit. Here, since paths through which the large amount of working fluid and the small amount of working fluid are discharged are not clear, in a case in which the large amount of working fluid and the small amount of working fluid are merged into a single drain line without dividing the large amount of working fluid and the small amount of working fluid, the large amount of working fluid, which is discharged to a drain tank when boom floating is performed, may cause pressure interference in a pilot line, and the interference may cause an erroneous operation when the boom is controlled.
In addition, pressure may be generated in spring chambers of a floating selection valve, a first floating valve, and a second floating valve due to a valve oil leakage, and the pressure may cause erroneous operations of the respective valves.
In addition, in a case a reverse load occurs on the boom cylinder when an unidirectional floating mode is selected, and the working fluid is held at a rod side of the boom cylinder, the working fluid is not replenished at the rod side of the boom cylinder, such that cavitation may occur, and as a result, rattling during a boom descending motion occurs when the boom is moved downward due to boom floating.